The invention concerns an optical imaging apparatus for the investigation of strongly scattering media, in particular biological tissue samples with at least one-dimensional position resolution which always includes the depth direction of a measured object, with a radiation source for the emission of light with short coherence lengths of less than 0.1 mm, with a device for splitting the low coherence light into two partial beams from which one is introduced into an object arm to the measured object and the other into a reference arm to a reflecting element and including a detector configuration onto which the partial beams from the reflecting element in the reference arm and from the measured object in the object arm can be guided, caused to come into interference with another, and detected.
An apparatus of this kind is, for example, known from the publication of Clivaz et al., OPTICS LETTERS Vol. 17, no. 1 (1992) 4-6. Similar devices are also disclosed in Takata et al., APPLIED OPTICS, Vol. 26, no. 9 (1987) 1603-1606, Takata et al., APPLIED PHYSICS LETTERS 59 (2) (1991) 143-145 or Swanson et al., OPTICS LETTERS Vol. 17, no. 2 (1992) 151-153.
Optical imaging in strongly scattering materials, for example in vivo tissue, is being increasingly used as an auxiliary means in medical research and diagnostics. "Classical" optical methods as, for example, microscopy cannot be utilized in strongly scattering materials since the mean free wave length, along which the light beam remains undeflected, assumes values less than 100 .mu.m. This means, that utilizing geometrical optics (e.g. confocal microscopy) it is not possible to achieve a penetration depth larger than 50 to 100 .mu.m since the diffuse scattered light portion increases exponentially and ruins the contrast of the images.
The most important methods for three-dimensional imaging up to depths of several millimeters in strongly scattering materials is based, on the one hand, on low coherence and, on the other hand, on short-time-interferometry. Both methods distinguish themselves essentially only through their light sources, whereby the emitted radiation in both cases exhibits short coherence lengths of typically 10 to 30 .mu.m. With low coherence interferometry an inexpensive incoherent light source (for example SLD or LED) is utilized, whereas for short-time interferometry an expensive (coherent) mode-locked laser which produces short light pulses is utilized.
The frequently utilized Michelson-interferometer is described briefly below. The collimated light of the light source is split by means of a beam-splitter and is incident in the reference arm on a mirror and in the object arm, after appropriate focusing, on the object. After reflection from a point of the object and from the mirror the light is joined together once more in the beam-splitter and, subsequently, is detected by, for example, a photodiode. When the optical path length of the object arm (defined by the mid-point of the beam-splitter and a point in the object) and that of the reference arm are identical up to a deviation on the order of the coherence length, both signals interfere at the detector. In order to easily detect an AC-signal (in contrast to a DC) the reference mirror is normally caused to oscillate at an appropriate frequency, along the optical axis with small deflections. During a mechanical displacement of the reference mirror along the optical axis, the depth of the object is sampled point for point. The strong scattering properties of the object do not influence theguality of the (one-dimensional) image since only the unscattered coherent components contribute to the AC-signal. However, due to the exponential fall-off of the coherent component of the signal only limited penetration depths are possible (see for example M. R. Hee et al., J. OPT. SOC. AM. B, Vol. 9, No. 6 (1992) 903-908).
The known devices are complicated and expensive due to the mechanically displaced reference mirror, are mechanically sensitive, and relatively slow since each point along the depth direction must be recorded in time sequence.
It is therefore the object of the present invention to introduce an optical apparatus of the above mentioned kind which is simple and cost effective, while being stable and insensitive with respect to mechanical interference, and which facilitates the taking of images in relatively rapid sequence.